1. Field of the Invention
The present invention relates to a projection exposing apparatus, and, more particularly to a projection exposing apparatus for manufacturing semiconductor integrated circuits or large-size liquid crystal substrates of a type having a mechanism for measuring the imaging characteristics of the projection optical system thereof.
2. Related Background Art
Technologies for correcting the projection optical system by measuring the imaging characteristics of the projection optical system in order to cause the formed mask pattern to be optimum have been disclosed in U.S. Pat. No. 4,629,313, Japanese Patent Laid-Open No. 63-306626 and U.S. Pat. No. 4,952,815.
The technology disclosed in U.S. Pat. No. 4,629,313 is arranged to use an apparatus comprising a test reticle having fine linear elements and a light receiving device disposed on the stage and receiving light passed through the test reticle through the fine reticle so that the characteristics of the projection optical system, for example, the inclination of the image surface or the curvature of field is obtained in accordance with change in an output signal from the light receiving device generated at the time of raising/lowering the stage along the Z-axis (the optical axial directional axis of the projection optical system). Thus, the imaging characteristics of the projection optical system are corrected in accordance with the result of the aforesaid measurement.
Each of the technologies disclosed in Japanese Patent Laid-Open No. 63-306626 and U.S. Pat. No. 4,952,815 is arranged to use an apparatus comprising a reference member having a reference mark and disposed on the stage and a test reticle having a special reference mark. By using the thus arranged structure, the reference member is irradiated with light from the lower surface and light beams, which have passed through the reference marks of the reference member and the test reticle, are received so that the imaging characteristics of the projection optical system are measured. The change in the imaging characteristics of the projection optical system taken place due to the rise of the temperature and the like caused from the absorption of exposing light is estimated so that the correction of the projection optical system is performed while estimating the change in the imaging characteristics which will be taken place at the time of the exposing operation in accordance with the result of the aforesaid estimation. With each of the aforesaid apparatus, the change in the imaging characteristics of the projection optical system, such as the inclination of the image surface or the curvature of field, taken place due to the thermal energy for use in the exposing operation can be corrected in real time.
However, the focal point position-detection by a through-the-lens method must use a reticle having a specific mark (a test reticle or an actual reticle which has a specific mark on the peripheral portion of the exposure region thereof). Therefore, the through put inevitably deteriorates because the test reticle is used at each measurement so as to measure the image forming characteristics and the image forming characteristics are corrected. What is worse, when the test reticle is used, the focus position at each pattern of the actual reticle cannot be measured immediately before the exposure. Therefore, the image forming characteristics of the projection optical system cannot be accurately detected immediately before the actual exposure.
Furthermore, the conventional apparatus encounters a problem in that the image forming characteristics cannot be measured in a state where the actual reticle is set thereto. Therefore, the change in the image forming characteristics of the projection optical system due to the change in the atmospheric pressure or the temperature during the exposure operation cannot be always accurately corrected. Therefore, in a case of the future 64M DRAM which must realize a line width of 0.3 to 0.4 .mu.m, a satisfactory focusing accuracy cannot be realized by the correction of the image forming characteristics by the conventional estimation control method. In addition, the through put deteriorates because the test reticles must be set at each exchange of the wafer in order to correct the image forming characteristics such as the curvature of field or the inclination of the image surface so as to correct the projection optical system.
In addition, a fact has been known that the astigmatism, the eccentricity or the spherical aberration affect the image forming characteristics of the projection optical system as well as the affections of the aforesaid curvature of field and the inclination of the image surface. However, since the conventional apparatus does not measure the astigmatism, the eccentricity or the spherical aberration, there is a fear that a desired focusing characteristics cannot be obtained if the width of the lie is considerably thinned. Incidently, the astigmatism is the difference between the focal point position between the medional direction and the sagittal direction and which is caused from the astigmatism.
In a case where the image forming characteristics of the projection optical system are detected by using the actual reticle in place of the test reticle, the focus state can be measured by using the mark formed in the periphery of the exposure region. However, the focal point detection cannot be performed in the central portion of the exposure region. Therefore, the curvature of field, the inclination of the image surface or the astigmatism cannot be measured.
A method in which test printing is performed by using the test reticle before the commencement of the exposure and the image forming characteristics are corrected has been employed. However, the change in the image forming characteristics cannot be tackled and the through put deteriorates if the test printing is performed for each wafer.
Also, a focusing mechanism (auto-focus mechanism) for indirectly setting the current exposure shot region of a photosensitive substrate within a range of the focal depth of the imaging plane of a projection optical system is known. In this indirect method, a measurement means for measuring the height of a stage with respect to the projection optical system is separately arranged, and the origin of the measurement means is aligned with an in-focus point obtained in advance using the above-mentioned direct method. Then, the height of the exposure surface of the photosensitive substrate is detected using the measurement means, and the exposure surface is indirectly guided to the in-focus point. For example, Japanese Patent Publication No. 1-41962 or U.S. Pat. No. 4,650,983 discloses, as an example of the measurement means for measuring the height of the stage, a mechanism for measuring the height of an exposure surface immediately below a projection optical system using an optical system of an oblique light incidence type fixed outside the projection optical system.
As a special example of the auto-focus mechanism, for example, Japanese Laid-Open Patent Application No. 57-212406 discloses a method for directly projecting an image of a special mark formed on a mask pattern surface onto the exposure surface of a photosensitive substrate, and detecting the projected image via a projection optical system and the mark, thereby directly discriminating an in-focus point.
Under such circumstances, recently, in the case of semiconductor memory devices requiring particularly high machining precision, a projection optical system using an i line of a wavelength of 365 nm, and having a focal depth of 1 .mu.m or less is used. In this case, precision as high as 0.1 .mu.m or less is normally required as an alignment precision of the in-focus point. For example, in a special projection exposure method utilizing an interference phenomenon of exposure light disclosed in Japanese Patent Publication No. 62-50811, a precision as very high as 0.05 .mu.m or less is required.
However, when the in-focus point is to be detected using the above-mentioned indirect method, the origin of the measurement means is deviated from the actual in-focus point of the projection optical system due to a change in imaging characteristics of the projection optical system caused by an environmental change, the type of a mask (reticle) to be used, aging, or the like, and it often becomes difficult to obtain such high precision. More specifically, in, e.g., the early step of the exposure process, even when the origin is aligned as a false in-focus point of the measurement means for measuring the height of the stage is controlled to fall within the range of the focal depth of the in-focus point obtained by the direct method (disclosed in the above-mentioned U.S. Pat. No. 4,629,313 or U.S. Pat. No. 5,117,254), if the imaging characteristics of the projection optical system change due to a drift of the ambient atmospheric pressure or temperature of the projection optical system during a continuous use of a projection exposing apparatus, the origin may undesirably fall outside the range of the focal depth of the imaging plane of the projection optical system.
In this case, if calibrations of the origin of the measurement means are performed by the direct method every time a photosensitive substrate is replaced, the origin correction time is prolonged, and the throughput is decreased.
In this connection, a predictive control correction method for predicting a variation in imaging characteristics of the projection optical system due to, e.g., a temperature rise caused by absorption of exposure light, and correcting the origin of the measurement means on the basis of the prediction result may be proposed. However, even in such a correction operation based on predictive control, a sufficient focusing precision cannot be expected when the required line width approaches 0.1 .mu.m like in, e.g., a 64-Mbit DRAM of the next generation. Recently, a projection exposing apparatus, such as an apparatus relating to manufacturing of semiconductors, must have an excellent image forming performance. Hitherto, an apparatus of the foregoing type has a projection optical system, the various aberrations of which have been precisely corrected, and which is mounted thereon. The check of the quantity of the aberration of the projection optical system has been usually performed by making use of a special mask. The foregoing special mask has an exclusive pattern for checking the aberration, the exclusive pattern being written thereon. The exclusive pattern is exposed to an experimental substrate, and then developed, the exposed pattern being then observed with a microscope or the like. As a result, the quantity of the aberration is checked. For example, checking of a spherical aberration is performed by obtaining a best focus position of different patterns having a plurality of line widths. Specifically, a stage is sequentially located in the direction of the optical axis, and the patterns having the plural line widths are exposed while sequentially moving, in the direction of the optical axis, an experimental substrate applied with a photosensitive agent, the exposed pattern being then developed. As a result, the patterns are formed on the substrate, the patterns having plural line widths which correspond to the distance from the projecting optical system in the direction of the optical axis. The line width of each of the patterns formed on the substrate and having the plural line widths is measured by making use of an electron microscope or the like. The actual dimensions of the measured line widths of the patterns and the distance from the optical axis of the projection optical system are plotted. If the middle point (an intermediate point between the maximum position of the substrate position at which a predetermined line width is maintained and the minimum position of the same) of the positions of the substrate at which a predetermined line width is maintained is determined to be the best focus, the best focus position of each line width is determined in accordance with the curve of plotted data. If a spherical aberration has taken place here, the best focus position of each line width can be displaced. The astigmatism can be checked by obtaining the difference in the best focus positions of the patterns formed in the different directions. Then, the comatic aberration can be checked by, for example, a method in which the difference between the line widths at the two ends of marks of plural, continuous and equal line widths.
By the foregoing methods, the respective aberrations are obtained to optimize the image forming characteristic of the projection optical system. For example, the intervals or the like of lens elements in the projection optical system are adjusted as to make finally each aberration to be lower than an allowable value. However, each aberration is not always constant but the same can be varied due to change in the temperature, change in the atmospheric pressure and a rise in the temperature of the projection optical system due to a fact that the projection optical system absorbs illumination light. In particular, the change of the temperature of the projection optical system occurring due to the absorption of illumination light causes the elements in the projection optical system to have a temperature distribution. As a result, an aberration change that cannot be ignored sometimes takes place. In the foregoing case, it is possible to check the quantity of aberration during the exposure operation by the foregoing method.
Accordingly, a method has been suggested which has an arrangement that the quantity of accumulation of exposure energy in the projection optical system is calculated, if the accumulation quantity is larger than a predetermined standard value, the exposure operation is temporarily stopped, and the exposure energy is not applied to the portion, the aberration of which has deteriorated. Specifically, the relationship between the quantity of aberration to be generated and the heating value to be accumulated in the projection optical system and the heat absorption characteristics of the projection optical system are previously obtained. At the time of an actual exposure operation, a numerical model of the previously obtained heat absorption characteristics, the mask permeability and a signal for opening/closing the shutter are used to calculate sequentially the heating value accumulated in the projection optical system as to perform calculations for the purpose of obtaining the quantity of the aberration generated. By temporarily stopping the exposure operation if the aberration quantity is larger than an allowable value, the heating value accumulated in the projection optical system is decayed, and also the aberration quantity is made smaller than the allowable value. Therefore, the exposure operation can be again performed. An exposure method of the foregoing type has been disclosed in, for example, Japanese Patent Application Laid-open No. 63-291417.
The foregoing conventional technology is able to obtain the aberration quantity as an estimated value and, accordingly, an actual quantity of the generated aberration cannot be obtained. In recent years, there has been suggested a technology for improving the resolving power by employing an annular illumination or an inclined illumination as disclosed in U.S. Ser. No. 791,138 in which the inclined light beams are applied from a plurality of directions. Further, a phase-shift mask as disclosed in Japanese Patent Publication No. 62-50811 has been disclosed. If the foregoing technology is employed, the distribution of the intensities of the illumination light beams in the projection optical system varies. Even if the illumination energy is the same, the quantity of the generated aberration becomes different depending upon the illumination conditions or a reticle. Therefore, it is difficult to predict the quantity of aberration to be generated. As a result, there arises a problem in that the conventional method of predicting the aberration quantity cannot be adapted to a desired exposure operation.